pkexec

pkexec — Execute a command as another user

Synopsis

pkexec [--version] [--disable-internal-agent] [--help]

pkexec [
--userusername
] PROGRAM [ ARGUMENTS ...]

DESCRIPTION

pkexec allows an authorized user to execute
PROGRAM as another user. If
PROGRAM is not specified, the default
shell will be run. If username is
not specified, then the program will be executed as the
administrative super user, root.

RETURN VALUE

Upon successful completion, the return value is the return value
of PROGRAM. If the calling process is
not authorized or an authorization could not be obtained through
authentication or an error occured, pkexec
exits with a return value of 127. If the authorization could not
be obtained because the user dismissed the authentication
dialog, pkexec exits with a return value of
126.

AUTHENTICATION AGENT

pkexec, like any other polkit application,
will use the authentication agent registered for the calling
process or session. However, if no authentication agent is
available, then pkexec will register its own
textual authentication agent. This behavior can be turned off by
passing the --disable-internal-agent option.

SECURITY NOTES

Executing a program as another user is a privileged
operation. By default the action to check for (see
the section called “ACTION AND AUTHORIZATIONS”) requires administrator
authentication. In addition, the authentication dialog presented
to the user will display the full path to the program to be
executed so the user is aware of what will happen.

The environment that PROGRAM will run
it, will be set to a minimal known and safe environment in order
to avoid injecting code
through LD_LIBRARY_PATH or similar
mechanisms. In addition the PKEXEC_UID
environment variable is set to the user id of the process
invoking pkexec. As a
result, pkexec will not by default allow you to run
X11 applications as another user since
the $DISPLAY and $XAUTHORITY
environment variables are not set. These two variables will be retained
if the org.freedesktop.policykit.exec.allow_gui annotation
on an action is set to a nonempty value; this is discouraged, though, and
should only be used for legacy programs.

Note that pkexec does no validation of
the ARGUMENTS passed
to PROGRAM. In the normal case (where
administrator authentication is required every
time pkexec is used), this is not a problem
since if the user is an administrator he might as well just
run pkexec bash to get root.

However, if an action is used for which the user can retain
authorization (or if the user is implicitly authorized) this
could be a security hole. Therefore, as a rule of thumb,
programs for which the default required authorization is
changed, should never implicitly trust user input (e.g. like any
other well-written suid program).

ACTION AND AUTHORIZATIONS

By default, the
org.freedesktop.policykit.exec action is
used. To use another action, use the
org.freedesktop.policykit.exec.path
annotation on an action with the value set to the full path of
the program. In addition to specifying the program, the
authentication message, description, icon and defaults can be
specified. If the org.freedesktop.policykit.exec.argv1
annotation is present, the action will only be picked if the
first argument to the program matches the value of the annotation.

Note that authentication messages may reference variables (see
the section called “VARIABLES”), for example
$(user) will be expanded to the value of the
user variable.

WRAPPER USAGE

To avoid modifying existing software to prefix their
command-line invocations with pkexec,
it's possible to use pkexec in a
she-bang wrapper
like this:

can be used to select the appropriate polkit action. Be careful
to get the latter annotation right, otherwise it will match any
pkexec invocation of
/usr/bin/python scripts.

VARIABLES

The following variables are set by
pkexec. They can be used in authorization
rules and messages shown in authentication dialogs:

program

Fully qualified path to the program to be executed.
Example: “/bin/cat”

command_line

The requested command-line (do not use this for any
security checks, it is not secure).
Example: “cat /srv/xyz/foobar”

user

The user name of the user to execute the program as.
Example: “davidz”

user.gecos

The full name of the user to execute the program as.
Example: “David Zeuthen”

user.display

A representation of the user to execute the program as
that is suitable for display in an authentication dialog.
Is typically set to a combination of the user name and the
full name.
Example: “David Zeuthen (davidz)”